1. Field of the Invention
The present invention relates to a hydrolyzing method and system for efficiently producing saccharides from biomasses, particularly cellulosic biomasses, used as raw materials.
2. Description of the Related Art
As part of biomass energy utilization, attempts have been made to obtain ethanol (bioethanol) by hydrolyzing cellulose or hemicellulose, which are major constituents of plants. Ethanol thus obtained is planned to be utilized as a fuel to be mixed into an automotive fuel or as an alternative fuel for gasoline.
Major constituents of plants include cellulose (a polymer of glucose, which is a C6 saccharide comprising six carbon atoms), hemicellulose (a polymer of a C5 saccharide comprising five carbon atoms and a C6 saccharide), lignin, starch, and the like. Ethanol is produced from saccharides, such as a C5 saccharide, C6 saccharide, and oligosaccharide which is a complex of these saccharides, used as raw materials, by the fermentation action of yeast fungi or the like.
Three methods of hydrolyzing a cellulosic biomass comprising cellulose, hemicellulose or the like into saccharides are about to be utilized industrially, which include: 1) a method of hydrolyzing such a biomass by the oxidative power of a strong acid, such as sulfuric acid; 2) a method of hydrolyzing such a biomass by yeast; and 3) a method utilizing the oxidative power of supercritical water, subcritical water or the like. However, the hydrolytic method 1) using the acid indispensably requires a treatment for neutralizing the added acid after hydrolysis of cellulose or hemicellulose into saccharides and before fermentation of the saccharides into ethanol because the added acid acts as an inhibitor against the fermentation by yeast or the like. The cost of such a treatment makes it difficult to put this method into practice in view of the economical aspect.
The outlook for industrial-scale realization of the hydrolyzing method 2) using yeast is still vague in view of the cost efficiency because an effective yeast for the method 2) has not been found yet and, if found, such a yeast is expected to incur a high production cost thereof, though the method 2) can be realized by a normal-temperature and normal-pressure process.
As the method 3) of hydrolyzing cellulose or the like into saccharides by using supercritical or subcritical water, patent document 1 has disclosed a method of producing water-insoluble polysaccharides, which is characterized by hydrolysis of cellulosic powder by bringing the powder into contact with pressurized hot water at 240 to 340° C. Patent document 2 has disclosed a method including: hydrolyzing biomass chips with hot water pressurized to a saturated vapor pressure or more at 140 to 230° C. for a predetermined time period to extract hemicellulose; and then conducting hydrolysis using pressurized hot water heated to a temperature not less than the cellulose hydrolyzig temperature to extract cellulose. Patent document 3 has disclosed a method of producing glucose and/or water-soluble cello-oligosaccharide, which is characterized in that cellulose having a mean polymerization degree of not less than 100 is hydrolyzed by the steps of: bringing the cellulose into contact with supercritical or subcritical water at a temperature of not lower than 250° C. and not higher than 450° C. and a pressure of not less than 15 Mpa and not more than 450 MPa for a time period of not less than 0.01 seconds and not more than 5 seconds; cooling the cellulose; and then bringing the cellulose into contact with subcritical water at a temperature of not lower than 250° C. and not higher than 350° C. and a pressure of not less than 15 Mpa and not more than 450 MPa for a time period of not less than 1 seconds and not more than 10 minutes.
On the other hand, patent document 4 has disclosed a method of treating a biomass-type waste, which includes: placing a subject for treatment containing a solvent comprising low-molecular-weight alcohol as a major component and the biomass-type waste into a closed vessel; and treating the subject by pressurizing and heating the interior of the closed vessel so that the low-molecular-weight alcohol reaches its supercritical condition. Also, patent document 5 has disclosed a method of hydrolyzing and liquefying a biomass, which includes treating a cellulosic biomass by using a mixed solvent prepared by adding 5-20% by volume of water to a C1 to C8 aliphatic alcohol under the supercritical or subcritical condition of the alcohol.                Patent document 1: Japanese Patent Provisional Publication No. 2000-186102        Patent document 2: Japanese Patent Provisional Publication No. 2002-59118        Patent document 3: Japanese Patent Provisional Publication No. 2003-212888        Patent document 4: Japanese Patent Provisional Publication No. 2001-170601        Patent document 5: Japanese Patent Provisional Publication No. 2005-296906        
As compared with the hydrolytic method using a strong acid, the method of hydrolytic saccharification of cellulose and hemicellulose as major constituents of a biomass by using high-temperature and high-pressure supercritical or subcritical water requires a lower processing cost and is a more environment friendly because this method does not require any acid neutralizing treatment. However, this method has a drawback that without cooling immediately after the completion of hydrolysis, saccharides produced thus far would degrade into organic acids or the like because the use of supercritical or subcritical water causes cellulose and hemicellulose to hydrolyze into saccharides completely in several seconds to several minutes by its strong oxidative power.
With a laboratory-scale small system for hydrolysis, it seems that such degradation can be prevented by rapidly cooling supercritical or subcritical water in the heating vessel. With an industrial-scale hydrolysis system, however, it is very difficult to cool a large amount of supercritical or subcritical water in a short time. For this reason, the cellulosic biomass hydrolysing method using high-temperature and high-pressure supercritical or subcritical water, when applied to a plant-scale system, will give a low yield of saccharides, which forms one of the factors that prevent this method from being put to practice.
In using a large amount of supercritical or subcritical water, the slurry has to be heated with a large amount of energy, which forms a factor raising the processing cost. The cellulosic biomass hydrolyzing method, which subjects a slurry containing alcohol or the like as a solvent to hydrolysis under a supercritical or subcritical condition, requires a very high vapor pressure, hence, requires a larger amount of energy and has to use a system having a high pressure resistance.
It is an object of the present invention to provide a method and system for hydrolyzing cellulose and/or hemicellulose contained in a biomass into monosaccharides and oligosaccharides (hereinafter will be referred to as “saccharides”) by using high-temperature and high-pressure water in a subcritical condition, which method and system is excellent in thermal efficiency and yields of saccharides.